A modern mobile device can function as a computer and as a cellular telephone. For example, the mobile device can perform traditional computer functions, such as executing application programs, storing various data, and displaying digital images. These functions can be performed in an application subsystem of the mobile device. The application subsystem can include an application processor, an application operating system, and various input/output devices.
When the mobile device functions as a cellular telephone, the mobile device can initiate and receive phone calls, identify cellular tower connections, and determine when and whether to switch cellular towers. These functions can be performed in a baseband subsystem of the mobile device. The baseband subsystem can include a baseband processor and a baseband operating system. The baseband processor can be an integrated circuit (IC) device (e.g., a Large Scale Integrated Circuit (LSI)) that performs communication functions. The baseband processor can include, for example, a code division multiple access (CDMA) unit. The baseband processor can be integrated with the application processor in a System-on-Chip (SoC). In general, the application subsystem can consume more power than the baseband subsystem when activated.